This invention relates to time assignment speech interpolation systems and more particularly to a data detector for such systems.
Because of the extremely high cost of communications transmission facilities, e.g., satellite channels and undersea transmission lines, the prior art has sought various means to maximize the efficiency of existing transmission facilities. One such system is known as a time assignment speech interpolation (TASI) system. In a typical TASI system, calls from n callers are transmitted across, for example n/2 transmission facilities to a remote location. At that location, the n/2 facilities are connected to n output speech channels. TASI systems operate on the assumption, verified as a statistical fact, that at any given time not all callers in a large group will wish to talk simultaneously. In fact, as a general rule, callers are actively talking less than half of the time the talker and the listener are interconnected. Accordingly, TASI systems may be defined as switching systems which interconnect talker and listener only when the talker is actively speaking, provided there is a transmission facility available at that time.
TASI systems are designed to multiplex only speech. The present invention relates to a TASI system designed to carry both speech and data interchangeably.
When an input channel carrying speech is switched to a transmission facility, there is some loss of information in a TASI system. Typically, 1% of the speech signal may be lost through "clipping" caused by the delay between the time speech is first detected and the time switching is accomplished. This is acceptable because a 1% loss in speech content is generally not objectionable to the listener. However, when a TASI system is used to transmit data, a 1% loss of information is unacceptable. Therefore, statistical switching must be disabled and a transmission facility dedicated to a caller who is transmitting data in a TASI system designed to carry both speech and data. In order to do this, a reliable data detector must be provided which can accurately distinguish data from speech. Typically, the standard data sets which are in abundant use display a full duplex format which contains energy in the range 2010 to 2240 Hz. However, not all data formats in use contain energy in this range. Therefore, it is not desirable simply to utilize a frequency discriminator to detect data within a particular frequency range.